Gallup, which has been tracking the share of uninsured Americans since 2008, outlined a few possible reasons for the recent spike.

One could be the departure of some insurers from the Affordable Care Act’s exchanges. Aetna, UnitedHealthcare, Anthem and Humana have all scaled back their presence on the exchanges this year. Gallup suspects that the decreased competition could mean higher prices for consumers — which may have encouraged people to opt out of insurance altogether.

The poll found that the percentage of adults who bought their own plans, like those offered by Obamacare, fell from 21.3% to 20% between the end of 2016 and the third quarter of 2017.

Another factor could be the uncertainty surrounding the GOP’s healthcare plans. Currently, Obamacare’s individual mandate penalizes people who don’t have insurance. If people think the Trump administration will do away with the penalty, they might not bother buying insurance at all.

Americans ages 35 to 64 were the most likely to drop insurance between the end of 2016 and the third quarter of this year. That demographic saw an increase of 1.8 percentage points in uninsured adults, Gallup found. The uninsured rate of young Americans, ages 18 to 25, went up by 1.5 percentage points.